Apple doesn't really have the best of search engines when it comes to Dashboard Widgets and honestly Widgets have an extremely limited usefulness imho.

Except for this little gem I found... Screenshot Plus. It's a perfect replacement for the standard Grab utility but with added features. Besides the standard screenshots of windows, full and partial screens and timed screenshots, it allows you to make screenshots of widgets. On top screenshots can be saved in different file formats (jpg, png, tiff, gif, pdf).